BULK CARBONATE δ13C RECORD FROM TIDWELL HOLLOW, ALABAMA: A SEARCH FOR THE GICE IN THE SOUTHEAST US

The Guttenberg Isotope Carbon Excursion (GICE), a ~3‰ positive carbon isotope excursion at the base of the Katian, is thought to reflect a perturbation in the global carbon cycle and possibly the onset of cooling and glaciation in the Late Ordovician. Documenting the regional and global variability of the GICE is a critical aspect in understanding the implications of the excursion for the global carbon cycle and paleoclimate during the Ordovician as well as for its use in chemostratigraphic correlation. To determine the regional expression of the GICE along the southern margin of the Laurentian carbonate platform we measured bulk carbonate δ¹³C and δ¹⁸O values from Tidwell Hollow in north-central Alabama. The ~230 m of section exposed at Tidwell Hollow is an excellent study site because it is well exposed, contains the Deicke and Millbrig k-bentonites, and is in a region where δ¹³C trends have not been studied.

Our carbonate δ¹³C curve shows an unexpected, ~2‰ negative excursion in upper P. undatus through P. tenuis conodont zones (the expected GICE interval) and no evidence of a positive excursion. Bulk carbonate δ¹³C values range from -2.7‰ to 1.5 ‰ with an average of -0.1‰; bulk δ¹⁸O values fluctuate widely, ranging from -7.3‰ to -1.3‰ and are thought to be compromised by diagenetic alteration. Variations in δ¹³C values correlate with variations in lithology whereas preliminary δ¹³C_org measurements show no consistent relationship with either lithology or carbonate δ¹³C values. The absence of the expected GICE signature suggests several possibilities, including: that local variables overwhelmed the global δ¹³C trend at Tidwell Hollow, that the appropriate interval is absent, or that the GICE is more a phenomenon of epicontinental seas than a global event. To distinguish among these possibilities and aid in paleoclimatic interpretations ongoing work includes detailed conodont biostratigraphy, δ¹³C_org measurements, and conodont apatite δ¹⁸O analyses.